Gas turbines known in the state of the art comprise a compressor, possibly divided in a low pressure compressor and a high pressure compressor. Furthermore, gas turbines have a combustor, where gas is mixed with compressed air. After exiting the combustor, the high energy gas stream then expands through the turbine, where energy is extracted to operate the compressor and produce mechanical work i.e. a torque.
The pressure turbine is usually divided into a high pressure turbine and a low pressure turbine, wherein the high pressure turbine can include more than one stage as well as the low pressure turbine includes typically several stages. Each stage typically includes a rotor and a stator. The rotor disc, also referred to as the turbine rotor, rotates about a centre line axis or a longitudinal axis of the gas turbine.
On the rotor disc, several blades are arranged and extend radially into the gas stream. These blades have to withstand high temperatures and high mechanical forces due to the rotation of the turbine rotor. Therefore, typical blades comprise a cooling system with a cooling air supply passage in a root portion of the blade. Cooling air is supplied to holes of an airfoil of the blade to cool the surface of the airfoil by creating a cooling film.
A stator is typically arranged upstream of the rotor. The stator comprises guide vanes. The guide vanes, also referred to as nozzle guide vanes, NGV, are static vanes for guiding the expending gas stream onto the airfoils of the blades of the rotor.
To prevent high temperature gas from entering the inner region of the turbine, the nozzle guide vanes as well as the blades comprise platforms forming a labyrinth-sealing.
Problems arise with extreme front or rear edges of platform regions of the nozzle guide vanes. The problem is that these regions are subject to hot gas temperatures but are difficult to cool. This sometimes causes oxidation during service.
Typical cooling methods for these extreme regions of NGV-platforms include impingement jets to the underside of the platform.
European patent application EP1 178 181 A2 shows a system for cooling the platform of a blade. Similar techniques can be used for cooling the platform of a nozzle guide vane. From European patent application EP 1 205 634 A2 it is known to embody a platform of a blade with a hollow cavity in fluid communication with a cooling air channel. The cavity is further provided with straight outlet holes directed to an adjacent band of a vane for cooling the latter. US patent application US 2009/0232660 A1 describes to provide a platform of a blade with internal cooling passages for cooling of the platform. These passages extend from cooling channels in a root of the blade to sides of the root and are arranged inclined and downward in respect to a longitudinal direction of the blade. However, the jets produced by such arrangements are not able to reach the extreme edge of the platform due to mechanical and seal features at these locations.